Single-ended self-oscillating dc-dc converter for intermittently energized load having VBE responsive current limit circuit

ABSTRACT

The converter delivers current to an intermittently energized load and includes a coupled inductor having a primary winding, a secondary winding and a feedback winding. A switching transistor is coupled in series with the primary winding of the inductor and switches between conductive and non-conductive states to control the flow of current through the primary winding. A positive drive circuit provides positive bias voltage to the switching transistor. A current limiting circuit senses the voltage across the base-emitter junction of the switching transistor to measure the primary winding current, removes the positive bias voltage when the primary winding current reaches a predetermined value, and thereby switches the transistor out of the conductive state into the non-conductive state. A semiconductor switch includes a control lead coupled to a voltage divider which applies a scaled voltage to the control lead causing the semiconductor switch to conduct when the switching transistor base-emitter voltage reaches a predetermined current limit voltage representative of a preselected switching transistor collector current limit. The semiconductor switch then shunts the positive bias voltage away from the switching transistor base and switches that transistor into the non-conductive state.

This is a Continuation application of U.S. patent application Ser. No.794,415, filed Nov. 4, 1985 now U.S. Pat. No. 4,682,081.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to power supplies, and more particularly to powersupplies which include a current-mode control system for sensing theinductor primary winding current level.

2. Description of the Prior Art

Prior art current-mode controlled power supplies utilize various typesfeedback systems to limit the peak current flow through the primarywinding of a coupled inductor. Generally a resistor or a currenttransformer is coupled in series with a switching transistor to sensethe current level in the primary winding. Both of these current levelsensing elements induce losses and reduce the power supply efficiencylevel.

Prior art current-mode controlled power supplies use various techniquesto transition the switching transistor from a conductive state into anon-conductive state when the primary winding current sensing circuitdetects that the maximum desired primary winding current level has beenreached. In accomplishing this switching function, some power supplycircuits remove the positive bias voltage from the switching transistor,while other circuits incorporate a negative bias supply which initiallyprovides a low level bias voltage which increases magnitude during theoutput capacitor charging cycle. When the negative bias voltage level islow, the switching circuit is comparatively unstable and inefficient.Toward the end of the capacitor charging cycle, the negative biasvoltage increases to a level which exceeds the switching transistoravalanche breakdown voltage producing highly inefficient operation andexcessive heat dissipation.

For the reasons discussed above, prior art current-mode controlled powersupplies typically operate at efficiency levels of from between fifty toabout seventy percent.

U.S. Pat. No. 4,321,507 (Bosnak) discloses a strobe power supplyutilizing a coupled inductor having a primary winding, a secondarywinding, a feedback winding and a drive winding. Current transformer T₂is inserted in the switching transistor emitter circuit to sense theprimary winding current level so that the switching transistor can beswitched from the conductive state into the non-conductive state at apredetermined maximum current level. This current-mode controlled powersupply controls the operation of the switching transistor by applyingand removing a positive bias voltage. It does not provide negative biasvoltage to the switching transistor in the non-conductive state.

U.S. Pat. No. 3,417,306 (Knak) discloses a strobe power supply circuitincluding an inductor having primary, secondary and feedback windings.The feedback winding is coupled to provide a negative bias voltage tothe switching transistor when the switching transistor is in thenon-conductive state. The magnitude of the negative bias voltage varieswith changes in the power supply output voltage.

In an article entitled "Current-Sensing IC Improves Regulation of PowerSupplies", Electronic Products, June 17, 1985, pages 77-82, Glenn Fritzdescribes a current-mode controlled power supply utilizing a specializedintegrated circuit in the power supply control logic. In the descriptionaccompanying FIG. 7 at page 82 of this article, the author indicatesthat the FIG. 7 power supply operates at a seventy percent efficiencylevel at full load. FIG. 1 at page 78 of this article depicts a powersupply utilizing a current sensing resistive feedback element. At page80, right hand column, the author indicates that a current transformermay be utilized as a primary winding current level measuring device.

SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to provide acurrent-mode controlled power supply which measures the inductor primarywinding current level by sensing switching transistor base voltage.

Another object of the present invention is to provide a current-modecontrolled power supply which provides an optimized, fixed negative biasvoltage to the power supply switching transistor regardless of the powersupply output voltage.

Yet another object of the present invention is to provide a current-modecontrolled power supply which includes positive drive means forproviding a positive bias voltage to the power supply switchingtransistor during the conductive state and negative drive means forproviding negative bias voltage to the power supply switching transistorduring the non-conductive state.

Still another object of the present invention is to provide acurrent-mode controlled power supply where the positive drive means andnegative drive means constitute independently controlled circuitelements which are automatically switched in and out of the power supplycircuit in response to each polarity reversal of the inductor feedbackwinding.

Still another object of the present invention is to provide acurrent-mode controlled power supply in which the N_(s) /N_(f) inductorturns ratio is controlled to provide a minimum magnitude negativepolarity feedback winding voltage which exceeds the optimum negativebias voltage level for the power supply switching transistor.

Still another object of the present invention is to provide acurrent-mode controlled power supply having an operating efficiencyexceeding eighty percent.

Briefly stated, and in accord with one embodiment of the invention, acurrent-mode controlled power supply delivers current to anintermittently energized load. The power supply includes a coupledinductor having a primary winding and a secondary winding. A switchingtransistor is coupled in series with the primary winding for switchingbetween conductive and non-conductive states to control the flow ofcurrent through the primary winding. Positive drive means is coupled tothe base of the transistor for providing a positive bias voltage to thetransistor to switch into and maintain the transistor in the conductivestate where energy is transferred into the inductor. Current limitingmeans is coupled to sense the base voltage of the transistor formeasuring the primary winding current and for removing the positive biasvoltage from the transistor when the primary winding current reaches apredetermined value to thereby switch the transistor out of theconductive state and into the non-conductive state. A load is coupled tothe secondary winding for receiving energy stored in the inductor afterthe transistor has been switched into the non-conductive state by thecurrent limiting means.

In another embodiment of the invention, the inductor further includes afeedback winding and the power supply includes negative drive means. Thenegative drive means is coupled between the feedback winding and thebase of the switching transistor to supply a constant negative biasvoltage to the transistor as energy is transferred from the inductorinto the load. The negative bias means provides a constant negative biasvoltage to the switching transistor even though the power supply outputvoltage and the voltage across the feedback winding varies.

DESCRIPTION OF THE DRAWINGS

The invention is pointed out with particularity in the appended claims.However, other objects and advantages together with the operation of theinvention may be better understood by reference to the followingdetailed description taken in connection with the followingillustrations, wherein:

FIGS. 1A and 1B represent an electrical schematic diagram of a preferredembodiment of the current-mode controlled power supply of the presentinvention. In the embodiment illustrated, the power supply includes anintermittently energized load in the form of a strobe flash lamp.

FIG. 2 represents a generalized block diagram depiction of the powersupply illustrated in FIGS. 1A and 1B.

FIG. 3 is a graph depicting the monotonically increasing collectorcurrent versus base voltage for the power supply switching transistor.

FIGS. 4A, 4B and 4C represent a series of voltage versus time plotswhere the time axis of each of the three graphs is identical. FIG. 4Aspecifically illustrates the manner in which the output or load voltageof the power supply varies with time. FIG. 4B represents a simultaneousplot of several parameters including feedback winding voltage, positivebias voltage and negative bias voltage provided by the power supply ofthe present invention. FIG. 4C plots the feedback winding voltage,positive bias voltage and negative bias voltage provided by a prior artpower supply.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In order to better illustrate the advantages of the invention and itscontributions to the art, a preferred hardware embodiment of theinvention will now be described in some detail.

Referring now to FIGS. 1A and 2, the current-mode controlled powersupply of the present invention receives a DC power input at the inputterminals designated as "+V" and "-V". In connection with the preferredembodiment of the invention to be described below, a 48 volt DC inputvoltage is assumed and will determine the specific component values andratings and the inductor turns ratios. The fuse, inductor, diodes,capacitors and resistor depicted to the left of the vertical dotted line10 perform standard power supply voltage conditioning and circuitprotection functions and will not be described in detail.

Coupled inductor 12 includes a primary winding 14, a center tappedsecondary winding 16 and a feedback winding 18. The black dot whichappears at one end of each of these windings represents a conventionalsymbol for indicating winding polarity. The number of turns utilized ineach winding of inductor 12 is abbreviated as follows: N_(p) =number ofturns in primary winding; N_(s) =number of turns in secondary winding;and N_(f) =number of turns in feedback winding.

A switching transistor designated both by reference number 20 and byreference letter "Q" is coupled in series with primary winding 14 ofinductor 12. When the power supply is initially energized, the inputvoltage +V is applied to primary winding 14 and to the collector ofswitching transistor Q. Resistor 22 and diode 24 form a part of apositive drive supply or positive drive means 26 and provide a currentflow path to supply a source of positive starting bias voltage to thebase of switching transistor Q. Diode 24 of positive drive means 26serves as isolating means for isolating positive drive means 26 from thebase of switching transistor Q when the switching transistor is in thenon-conductive state.

In response to the initial starting bias voltage, switching transistor Qswitches from the non-conductive state into the conductive state wherethe level of current through primary winding 14 of coupled inductor 12begins to increase from a zero level. Commencement of current flowthrough primary winding 14 induces a positive voltage in feedbackwinding output terminal 28 to the input of positive drive means 26. Thispositive feedback voltage flows through resistors 30 and 32 and throughdiodes 34 and 24 and provides a higher level fixed positive bias voltageto the base of switching transistor Q as long as transistor Q remains inthe conductive state. While Q remains in the conductive state, energyfrom the DC power input source is transferred into inductor 12.

Current limiting means or current limiting circuit 36 includes a voltageconductor 38 which is coupled to sense the base voltage on switchingtransistor Q. As a result of the monotonically increasing relationshipbetween the switching transistor collector current and base voltage asillustrated in FIG. 3, a predictable, fixed relationship exists betweenbase voltage and collector current. In the current-mode controlled powersupply system of the present invention, it is necessary to transitionswitching transistor Q from the conductive state into the non-conductivestate when the switching transistor collector current reaches athreshold value. This threshold value is set at a level which preventssaturation of the magnetic core of inductor 12 and also protectsswitching transistor Q from potentially destructive overcurrentconditions. Once the appropriate limiting collector current has beenselected for switching transistor Q, the FIG. 3 collector current tobase voltage transfer curve yields a base voltage which corresponds tothe desired collector current limit. Current limiting means 36 istherefore configured to remove the positive bias voltage produced bypositive drive means 26 when the primary winding current reaches apredetermined value by sensing the base voltage of switching transistorQ. Removal of this positive bias voltage switches transistor Q out ofthe conductive state and into the non-conductive state.

Current limiting means 36 includes voltage scaling means in the form ofa voltage divider network consisting of resistors 40 and 42. The outputof this voltage scaling means is coupled to the input terminal 44 ofswitching means or semiconductor switch 46 consisting of a firsttransistor switch 48 and a second transistor switch 50. Resistors 40 and42 are selected to produce a specific scaled output voltage on controllead or input terminal 44 of switchig means 46 when the selected peakswitching transistor base voltage is sensed at voltage conductor 38. Atthis threshold voltage level, first switching transistor 48 is switchedinto the "on" state, activating second transistor switch 50. The emitterterminal of second transistor switch 50 is coupled between resistor 32and diode 24 and when activated shunts to ground the positive biasvoltage produced by positive drive means 26. Removal of this positivebias voltage from the base of switching transistor Q immediatelycommences the process of switching switching transistor Q out of theconductive state and into the non-conductive state.

As soon as the primary winding current level begins to decrease as aresult of removal of the positive bias from switching transistor Q, thepolarity on output terminal 28 of feedback winding 18 switches from apositive voltage to a negative voltage as indicated by the voltagepolarity symbols in FIGS. 1A and 2. Feedback winding 18 is designated toinclude a sufficient number of turns such that this instantaneousnegative feedback voltage is of a sufficient magnitude to energizenegative drive means 52 to immediately provide a specific, fixednegative bias voltage to the input or base terminal of switchingtransistor Q. In the preferred embodiment of the invention, negativedrive supply or negative drive means 52 is configured as a voltageregulator in the form of a series pass voltage regulator. This voltageregulator includes a zener diode 54, a transistor 56, a resistor 58 anda diode 60. The cathode of zener diode 54 is coupled to the positivepolarity terminal of feedback winding 18 as illustrated in FIG. 1A.

The removal of the positive bias voltage from the base terminal ofswitching transistor Q and the nearly simultaneous provision of anoptimized, fixed negative biased voltage on that same terminal rapidlydepletes the base-emitter stored charge on switching transistor Q andcauses this transistor to switch from the conductive state into thenon-conductive state at a speed substantially faster than if either nonegative bias voltage were applied or if a negative bias voltage havinga magnitude less than an optimized fixed value were applied. Switchingtransistor Q therefore switches at speeds comparable to substantiallymore expensive, higher grade switching transistors, permittingsignificant reductions in the overall cost of the power supply of thepresent invention. Negative bias also increases the collector to emitterbreakdown voltage of transistor Q, thereby increasing the ability of thecircuit to accept transient voltages.

At a predetermined time after switching transistor Q has been in thenon-conductive state, the current through primary winding 14 drops to alevel which causes the negative output voltage from feedback winding 18to drop below the minimum voltage input requirement for negative drivemeans 52. When this happens, the fixed negative bias voltage fromnegative drive means 52 is removed from the base of switching transistorQ. At that same time, current flow through resistor 22 and diode 24 ofpositive drive means 26 positively biases the base of switchingtransistor Q and once again switches transistor Q into the conductivestate as described above.

During the entire time that negative drive means 52 maintains switchingtransistor Q in the non-conductive state, the energy previously storedin inductor 12 during the conductive state is transferred from primarywinding 14 into secondary winding 16. The energy thus transferred flowsthrough diodes 62 and 64 and provides an incremental charge to energystorage capacitors 66 and 68. During each sequential interval thatswitching transistor Q is in the non-conductive state, the incrementalcharge level on capacitors 66 and 68 increases such that the resultingpower supply output or load voltage increases from a zero level to apredetermined desired output level. A plot of load voltage versus timeis illustrated in FIG. 4A.

Referring now to FIGS. 4A, 4B and 4C, the operation of the presentinvention and the advantages over prior art systems will be explained indetail.

FIG. 4B plots the voltage produced at output terminal 28 of feedbackwinding 18 versus time. When the power supply is initially energized asdescribed above, the positive voltages generated at output terminal 28of feedback winding 18 energizes positive drive means 26 to provide theinitial pulse of a positive bias voltage designated by reference number70. When current limiting means 36 disconnects the positive bias voltagefrom switching transistor Q, the output terminal of feedback winding 18generates the negative feedback winding voltage designated by referencenumber 72. This negative feedback winding voltage provides the inputpower source to negative drive means 52 which generates a regulated,constant negative bias voltage designated by reference number 74.Although the voltage level designated by reference number 74 is depictedas a continuous line, it should be understood that the negative biasoutput signal from negative drive means 52 will occur in pulses existingonly during each negative polarity feedback winding output pulse of thetype designated by reference number 72.

In designing inductor 12, it is necessary to configure the N_(s) /N_(f)turns ratio such that the first negative feedback winding voltage pulsedesignated by reference number 72 exceeds the desired fixed negativebias voltage level designated by reference number 74. If thisrelationship is not maintained, negative drive means 52 will not operatesince the input voltage will be less than the breakdown voltage of zenerdiode 54.

In FIG. 4B, the fixed negative bias voltage level 74 generated bynegative drive means 52 is illustrated as being a predetermined levelbelow the avalanche voltage designated by reference number 76. If anegative bias voltage equal to or greater than avalanche voltage 76 isapplied to switching transistor Q, the transistor will immediatelytransition into the avalanche breakdown region where excessive currentlevels and excessive transistor heating levels are experienced which maylead to transistor failure. On the other hand, it is desirable to havesufficiently high level negative bias voltage to provide for rapidswitching from the conductive into the non-conductive state to enhancecircuit stability to increase transient resistance, to reduce transistorheating and to generally maximize operating efficiency. The negativebias voltage level designated by reference number 74 has therefore beenreferred to as an "optimized" negative bias voltage since the negativebias voltage can easily be controlled by an appropriate negative drivemeans component selector in combination with an appropriate selection ofthe N_(s) /N_(f) turns ratio. Generally, this "optimized," fixednegative bias voltage will be chosen to be close to but slightly belowthe avalance breakdown voltage of switching transistor Q. In the 48 VDCembodiment of the invention, negative drive means 52 produces a constantnegative five volt bias level.

Provision of a controlled, constant magnitude negative bias voltage inprior art current-mode controlled power supplies has not previously beenpossible. As illustrated in FIG. 4C, prior art current-mode controlledpower supplies typically provide a monotonically increasing negativebias voltage. The initial negative bias voltage illustrated by referencenumber 78 is of a very low level which potentially causes circuitinstability, power loss, heating and generally inefficient operation.Only after numerous switching cycles do prior art current-modecontrolled power supply systems attain the optimized fixed negative biaslevel indicated by reference number 80. This increasing magnitudenegative bias voltage is proportional to the load voltage depicted inFIG. 4A and continues to increase with time. At the time designated byreference number 82, the monotonically increasing negative bias voltageprovided to the switching transistor exceeds the avalanche breakdownvoltage potentially causing damage to the transistor and renderingcircuit operation highly inefficient. With these prior art systems, theN_(s) /N_(f) turns ratio is controlled to limit the time to the right ofreference number 82 where switching transistor Q operates in theavalanche breakdown mode. A direct result of this design criteria isthat the monotonically increasing negative bias voltage produced betweentime zero and the time interval designated by reference number 80 isless than the optimum negative bias voltage operating level. Only duringthe time interval between times designated by reference numbers 80 and82 does the prior art current-mode controlled power supply providenegative bias voltage levels near the "optimized" level provided by thepresent invention.

The current-mode controlled power supply depicted in FIG. 1A can beutilized to energize a variety of different intermittently energizedloads. In FIG. 1B, the power supply of the present invention is coupledto a conventional strobe lamp system. The circuitry depicted in FIG. 1Bperforms the function of triggering and then energizing strobe lamp 84after the output voltage of the FIG. 1A power supply reaches apredetermined desired output voltage. In the embodiment depicted, strobelamp 84 is energized after the power supply output voltage has reachedfive hundred seventy volts. Network 86 includes a resistor 88 and azener diode 90 and provides charging current to capacitor 92 which formsa part of trigger pulse generator circuit 94. Timing generator 96periodically generates an output pulse which actuates SCR 98 anddischarges capacitor 92 through trigger transformer 100. Triggertransformer 100 generates a high voltage output pulse which ionizes theXenon gas in the strobe lamp which permits the energy stored incapacitors 66 and 68 to flow through strobe lamp 84 and create a brightflash of light.

Passage of the high level strobe flash current through series coupleddiodes 102 and 104 generates a voltage pulse on the order of three voltson voltage conductor 106 which is routed through resistor 108 and diode110 to the voltage input terminal 44 of switching transistor 48. Thisvoltage pulse actuates current limiting means 36 and either transitionsswitching transistor Q into the non-conductive state or maintains it inthe non-conductive state for the duration of the current pulse throughstrobe lamp 84. This disable signal prevents the power supply fromcontinuously applying energy to the lamp, thereby preventing continuousionization of the lamp.

The output of voltage divider network 112 is coupled to disablingcircuit 114. Potentiometer 116 is adjusted so that disabling circuit 114is actuated to generate an output pulse when the load voltage hasreached the desired level which in the preferred embodiment beingdescribed is equal to five hundred seventy volts DC. The output pulsefrom disabling circuit 114 is coupled to input terminal 44 of switchingtransistor 48 and holds switching transistor Q in the non-conductivestate.

Timing generator 96 periodically generates an output pulse whichtriggers SCR 98 as described above. Potentiometer 118 controls the pulserepetition rate of timing generator 96 and sets the flash rate of strobelamp 84. In the preferred embodiment of the invention, potentiometer 116is set to cause the power supply to generate a five hundred seventy voltVDC output voltage while potentiometer 118 is set to create an outputpulse every 0.75 seconds.

Coupled inductor 12 as used in the specific embodiment of the presentinvention described above is fabricated according to the followingspecifications: Bobbin-Cosmo No. 2-5570 with pin numbers 1, 4, 7, 8 and10 in place; Pot Core-Stackpole Code 55-0762, grade C/24B 0244, gapped.015 inches; windings fabricated from Phelps Dodge Nyleze film coatedcopper wire; secondary winding--124 turns center tapped, #29 AWG,inductance=6.8 mH; primary winding--53 turns, #22 AWG, inductance=1.3mH; feedback winding--6 turns #26 AWG, inductance=28 μH; winding order:first winding-secondary; second winding-feedback; third winding-primary.

The following table recites the component values utilized in fabricatingthe power supply described above and illustrated in FIG. 1A:

    ______________________________________                                        REFERENCE NO.      PART NO./VALUE                                             ______________________________________                                        Transistors                                                                   56                 MPSA92                                                     50                 D45C1                                                      48                 MPS6531                                                    Q                  MJ16002                                                    Diodes                                                                        24 & 34            1N4006                                                     60, 62 & 64        FR155                                                      54                 1N4733                                                     Resistors                                                                     30                 1kΩ                                                  32                 10kΩ                                                 22                 15kΩ                                                 40                 5.6kΩ                                                42                 10kΩ                                                 Capacitors                                                                    66 & 68            200 μF, 300 volts                                       ______________________________________                                    

The power supply of the present invention achieves a substantiallyenhanced efficiency level in comparison to commonly available prior artunits. For example, when the power supply of the present invention iscoupled to the strobe system depicted in FIG. 1B, an efficiency leveldetermined by the ratio of the power output versus the power input isequal to eighty-seven percent. Utilization of a prior art power supplyof the type illustrated in U.S. Pat. No. 4,321,507 produces an operatingefficiency of fifty-eight percent. Comparing the eighty-seven percentefficiency of the present invention with the fifty-eight percentefficiency of the prior art device identified above indicates that thepresent invention achieves a fifty percent increase in operatingefficiency.

This substantially enhanced power supply operating efficiency isachieved as a result of the cooperative relationship of severaldifferent elements of the present invention. The prior art strobe powersupply illustrated in U.S. Pat. No. 4,321,507 utilizes a currenttransformer having a magnetic core and a primary winding which iscoupled in series with a switching transistor. Such current transformerlosses are eliminated with the present invention which measures theinductor primary winding current level by sensing the switchingtransistor base voltage. In addition, the current limiting means used inthe present invention provides precisely accurate, repetitive operation.This concise control peak inductor primary current prevents saturationof the inductor magnetic core and provides a controllable safety marginfor sensitive active devices such as switching transistor Q. The amountof energy transferred into the primary winding 14 of inductor 12 canalso be precisely controlled on a repeatable basis to enhance theoverall operating efficiency.

The negative drive means of the present invention provides an optimum,fixed negative bias voltage during the entire charging operation. Theavailability of the optimized bias voltage allows the rapid depletion ofstored charge from the base-emitter junction of the switching transistorwithout causing base-emitter junction avalanche. This increases thetransistor switching speed and collector-emitter breakdown voltagewithout overly increasing transistor temperature rise and withoutwasting stored transformer energy as energy is transferred to thesecondary load. This feature greatly enhances efficiency and reliabilityand also allows the use of lower speed switching transistors to decreasecost.

A very unique aspect of the present invention resides in the fact thatpositive drive means 26 is configured such that it receives energy fromoutput terminal 28 of feedback winding 18 only when this output terminalprovides a positive polarity output voltage. In a similar manner,negative drive means 52 is configured such that it receives energy fromfeedback winding 18 only when a negative output voltage is available onoutput terminal 28 of feedback winding 18. The power supply of thepresent invention therefore utilizes the automatically switched voltagepolarities available at output terminal 28 of feedback winding 18 toprovide a switching function for switching positive drive means 26 intothe circuit while simultaneously switching out negative drive means 52and for subsequently switching out positive drive means 26 whileswitching in negative drive means 52. This automatic, synchronizedswitching feature of the present invention ensures that positive drivemeans 26 will be coupled to the base of switching transistor Q when thattransistor is in the conductive state and that negative drive means 52will be coupled to the base of switching transistor Q only when theswitching transistor is in the non-conductive state. As a result, ahighly efficient, low cost current-mode controlled power supply systemis achieved while utilizing only a minimum number of discrete circuitcomponents.

It will be apparent to those skilled in the art that the disclosedcurrent mode controlled power supply may be modified in numerous waysand may assume many embodiments other than the preferred formspecifically set out and described above. For example, numerousdifferent types of regulator circuits other than the series passregulator circuit depicted in FIG. 1A may be utilized to function asnegative drive means 52. Various different configurations of currentlimiting means 36 other than the specific two transistor circuitdepicted would also be readily apparent to one of ordinary skill in theart. Positive drive means 26 could take numerous differentconfigurations which would readily be apparent to one of ordinary skillin the art. Although the power supply of the present invention has beendepicted as energizing a strobe flash system, numerous other differenttypes of loads could be coupled to the output terminals of the powersupply depicted in FIG. 1A. Accordingly, it is intended by the appendedclaims to cover all such modifications of the invention which fallwithin the true spirit and scope of the invention.

I claim:
 1. Apparatus for delivering current to an intermittentlyenergized load comprising:a. a coupled inductor including a primarywinding a secondary winding and a feedback winding; b. a switchingtransistor having base, collector and emitter terminals and abase-emitter junction and being coupled in series with said primarywinding for switching between conductive and non-conductive states tocontrol the flow of current through said primary winding; c. a positivedrive supply coupled between said feedback winding and the base of saidswitching transistor and energized by said feedback winding forproviding a positive bias voltage to said switching transistor to switchinto and maintain said switching transistor in the conductive state totransfer energy into said inductor; d. a current limiting circuit forremoving the positive bias voltage from said switching transistor whenthe primary winding current reaches a predetermined value to therebyswitch said switching transistor out of the conductive state and intothe non-conductive state, said current limiting circuit includingi. asemiconductor switch having a control lead with a threshold voltage; ii.a voltage divider coupled across the base-emitter junction of saidswitching transistor for applying a scaled output voltage to the controllead of said semiconductor switch to cause said semiconductor switch toconduct when the base-emitter voltage of said switching transistorreaches a predetermined current limit voltage representative of apreselected switching transistor collector current limit; iii. saidsemiconductor switch shunting the positive bias voltage away from thebase of said switching transistor when the voltage on the control leadrises above the threshold voltage to switch said transistor out of theconductive state and into the non-conductive state; and e. a loadcoupled to said secondary winding for receiving energy stored in saidinductor after said switching transistor is switched into thenon-conductive state by said current limiting circuit.
 2. The apparatusof claim 1 wherein said semiconductor switch comprises a firsttransistor switch having base, collector and emitter terminals andwherein the base terminal of said first transistor switch functions asthe control lead of said semiconductor switch.
 3. The apparatus of claim1 further including positive and negative voltage input terminals andwherein the emitter terminal of said switching transistor is coupleddirectly to the negative voltage input terminal.
 4. The apparatus ofclaim 1 wherein said positive drive supply further includes means forisolating said positive drive supply from the base of said switchingtransistor while said transistor is maintained in the non-conductivestate.
 5. The apparatus of claim 4 wherein said isolating means includesa diode.
 6. The apparatus of claim 1 wherein said load includes a strobeflash lamp.
 7. The apparatus of claim 6 further including a sensor forsensing the output voltage on said secondary winding and for activatingsaid current limiting circuit when the output voltage reaches apredetermined level.
 8. The apparatus of claim 6 further including atrigger pulse generator and a timing generator coupled to said triggerpulse generator for periodically energizing said strobe flash lamp. 9.The apparatus of claim 1 further including a negative drive supplycoupled between said feedback winding and the base of said switchingtransistor for supplying a constant negative bias voltage to the base ofsaid switching transistor independent of variations in the voltageacross said feedback winding.
 10. The apparatus of claim 9 wherein theconstant negative bias voltage from said negative drive supply is set ata level less than the base-emitter avalanche voltage of said switchingtransistor.
 11. The apparatus of claim 10 wherein said negative drivesupply includes a voltage regulator.
 12. The apparatus of claim 11wherein the turns ratio of said secondary winding with respect to saidfeedback winding yields a feedback winding voltage which is alwaysgreater than said negative drive supply.
 13. The apparatus of claim 9wherein said positive drive supply further includes means for isolatingsaid positive drive supply from the base of said switching transistorwhile said negative drive supply provides the negative bias voltage tosaid switching transistor.
 14. The apparatus of claim 13 wherein saidisolating means includes a diode.
 15. Apparatus for delivering currentto an intermittently energized load comprising:a. a coupled inductorincluding a primary winding a secondary winding and a feedback winding;b. a switching transistor having base, collector and emitter terminalsand a base-emitter junction and being coupled in series with saidprimary winding for switching between conductive and non-conductivestates to control the flow of current through said primary winding; c. astarting voltage source coupled to the base of said switching transistorfor continuously applying a starting bias voltage to said switchingtransistor to periodically switch said switching transistor into theconductive state; d. a positive drive supply coupled between saidfeedback winding and the base of said switching transistor and energizedby said feedback winding for providing a positive bias voltage to saidswitching transistor to maintain said switching transistor in theconductive state to transfer energy into said inductor, the magnitude ofthe positive bias voltage exceeding the magnitude of the starting biasvoltage; e. a current limiting circuit for removing the positive biasvoltage from said switching transistor when the primary winding currentreaches a predetermined value to thereby switch said switchingtransistor out of the conductive state and into the non-conductivestate, said current limiting circuit includingi. a semiconductor switchhaving a control lead with a threshold voltage; ii. a voltage dividercoupled across the base-emitter junction of said switching transistorfor applying a scaled output voltage to the control lead of saidsemiconductor switch to cause said semiconductor switch to conduct whenthe base-emitter voltage of said switching transistor reaches apredetermined current limit voltage representative of a preselectedswitching transistor collector current limit; iii. said semiconductorswitch shunting both the starting bias voltage and the positive biasvoltage away from the base of said switching transistor when the voltageon the control lead rises above the threshold voltage to switch saidtransistor out of the conductive state and into the non-conductivestate; and f. a load coupled to said secondary winding for receivingenergy stored in said inductor after said switching transistor isswitched into the non-conductive state by said current limiting circuit.16. The apparatus of claim 15 wherein said apparatus includes a voltageinput lead and wherein said starting voltage source includes a powerinput terminal coupled to said voltage input lead.
 17. The apparatus ofclaim 15 further including a negative drive supply coupled between saidfeedback winding and the base of said switching transistor for supplyinga negative bias voltage to the base of said transistor as energy istransferred from said inductor into said load.
 18. The apparatus ofclaim 17 wherein said negative drive supply supplies a constant negativebias voltage to the base of said switching transistor as energy istransferred from said inductor into said load, wherein the negative biasvoltage remains constant as the voltage across said feedback windingvaries.
 19. The apparatus of claim 18 wherein said semiconductor switchcomprises a first transistor switch having base, collector and emitterterminals and wherein the base terminal of said first transistor switchfunctions as the control lead of said semiconductor switch.
 20. Theapparatus of claim 19 wherein said load includes a strobe flash lamp.